Check out monoprice.com. I just purchased 3 wall chargers and 2 car chargers for less than $9 after shipping. The cost of the chargers by themselves was $5.87.
These are for a different phone, but are mini usb. I have not received my order so I cant verify that they work, but I dont see why they would not? You will be hard pressed to find better prices with the same level of customer service. Its one of my favorites sites.